1. Field of the Invention
Embodiments of the invention relate to methods, systems, and apparatus for recycling deoiled water for heavy oil production.
2. Background of the Related Art
Oil may be extracted from underground reserves using a number of processes. One of those processes is steam-assisted gravity drainage (SAGD). In a typical SAGD process, steam is used for downhole injection to increase the flowability of oil and allow it to be withdrawn from a formation. This process generates a large volume of water that includes oil and other contaminants that flows to the surface through a producing well. It is important for the process to recycle this water for two primary reasons.
First, the boiler that produces the steam for the SAGD process requires a large feed water flow rate, typically 3 to 6 times the volume of oil being produced. For reasons of water conservation and to minimize the impact on certain external water resources (such as fresh water and brackish water) this boiler feed water requirement can typically be satisfied to a high degree (>90%) by recycling the water returning to the surface in the producing well.
Second, the water returning to the surface in the producing well contains contaminants that do not allow this water stream to simply be discharged to the environment.
For reasons of expense and potential environmental impact associated with the extraction, use, and disposal of large amounts of water, it is desirable to re-use water in the SAGD process.
One component that may be used in recycling of SAGD produced water is an evaporator. Co-current vertical-tube falling-film (VTFF) evaporators have historically been used exclusively for this application. Reasons for their common use include 1) the ability to provide a large amount of useable heat transfer surface area in a single vessel, 2) the large amount of heat transfer surface area allows the evaporation capacity per evaporator to be maximized, 3) the vertical design effectively minimizes overall footprint (plant space requirement), 4) the high overall heat transfer coefficient (HTC) that is achievable compared to other evaporator styles, 5) the relatively low power consumption requirement (per gallon of distillate produced) compared to other evaporator styles (achieved with high HTC) and 6) the co-current evaporator is naturally designed with a large sump volume which has traditionally been thought a requirement for water chemistry management.
Co-current VTFF evaporators work by continuously circulating a large volumetric flow rate of water from the evaporator sump to the top-head where the brine is distributed into the heat transfer surface (tubes) as a falling film. As steam condenses on the outside of the tubes, a portion of the falling film is vaporized and travels downward with the falling film where the vapor is disengaged and flows out of the evaporator, (refer to FIGS. 2 and 3).
The co-current evaporator is physically very tall and can extend as high as 150 feet above grade. Approximately one-third of this height is due to the heat transfer surface requirements (vertical tube length) while the balance of the extreme height is mostly due to the significant requirements of the evaporator sump. The evaporator sump is required to be very large for three primary reasons. First, sufficient volume must be provided (below the bottom-tube sheet and above the sump liquid level) to allow the vapor to be released from the evaporator tubes and to be released out of the evaporator vessel. Second, a certain minimum sump volume is required to allow the recirculation pump to be initially started without starving the pump. The liquid level in the evaporator sump will rapidly drop upon initial start-up of the recirculation pump as volume is taken from the sump and fills the recirculation piping, fills the top-head and coats the evaporator tubes. The sump must be large enough to allow this change in volume without immediately starving the recirculation pump. Note that the opposite is also true upon a recirculation pump stop: the water being held-up in the evaporator and recirculation piping will collect very quickly in the evaporator sump immediately after the recirculation pump is stopped. The sump volume must be large and sufficient to accommodate both of these common transient operating scenarios. This is inherent in the design of any co-current VTFF evaporator.
Finally, the sump must also contain a large volume to achieve a chemical equilibrium prior to circulating the brine water to the top-head and distributing onto the heat transfer surface. If chemical equilibrium is not achieved prior to circulation onto the tubes, the dissolved components will tend to precipitate and will form a scale on the evaporator tubes causing the heat transfer efficiency to diminish rapidly. The evaporator sump volume is a critical scale minimization parameter.
The importance of evaporator sump volume for residence time to achieve chemical equilibrium is implicitly understood in technology currently applied to SAGD application. For example, Canadian Patent No. CA2307819, to Heins (assignee Ionics, Incorporated) (the “Heins patent”) reports a process that will raise the pH of the evaporator concentrated brine internal to the evaporator. In such a process where hardness levels typically range from 5 mg/L to 25 mg/L (as CaCO3), the hardness will precipitate from solution as calcium carbonate and magnesium hydroxide. Such reactions require an extended length of time to arrive at equilibrium and come to completion; this can be as long as 4-10 minutes depending on operating conditions. If such a residence time is not provided in the evaporator sump, the precipitate from this softening reaction will take place while at the tube surface which causes scale and suboptimal evaporator performance. Even evaporators provided with long residence times in the evaporator sump will still scale at a certain frequency because it is often impractical to provide sumps large enough to ensure the brine exiting the sump is truly in equilibrium.
Another example of the importance of large evaporator sump volume can be seen in the “Seeded Slurry” or “Sorption Slurry” processes that rely on certain chemical species to preferentially form with an already suspended chemical species. In such systems, the evaporator designer must size the evaporator with a large sump to ensure there exist ample seed surface sites to allow the majority of the precipitating species to attach onto the parent solids. One skilled in the art would also recognize that the method reported by the Heins patent requires a large evaporator sump volume for the seeded slurry process to ensure sufficient seed surface sites exist to lower evaporator heat transfer surface scaling. The method reported by United States Patent Application Publication No. US2009/0056945, to Minnich, et al. (assignee HPD, LLC) would also require a large evaporator sump to lower evaporator heat transfer surface scaling. Seeded Slurry processes do not prevent evaporator scaling, they merely lower the scaling rate. This type of treatment method offers suboptimal performance since the evaporator will be unavailable to the process at regular intervals for cleaning. The heat transfer efficiency is also suboptimal since the scale that accumulates causes the mechanical vapor compression (MVC) process to consume more electrical power at a given capacity.
The aforementioned reasons have been the rationale behind the selection of co-current VTFF evaporators currently applied in this technical field. However, the technology currently applied has several distinct deficiencies that cause the delivered solutions to be undesirable in some respects. The first deficiency we note is a suboptimal process performance of the evaporators caused by heat transfer scaling/fouling. Even with the provision of a large sump volume, it is impractical to provide a sump with a volume truly large enough to allow the water chemistry to achieve equilibrium in the evaporator sump. Thus, complete mitigation of scaling/fouling is not achieved, which causes electrical power consumption to increase (in terms of kW/evaporation rate) and also requires significant process downtimes for maintenance and cleaning (heat transfer restoration).
Another recognized deficiency is the challenges and costs inherent in the fabrication, transportation (logistics) and installation/construction of evaporators of such immense dimensions. Co-current falling-film evaporators have total heights as tall as 150 feet and weigh several hundred thousands of pounds. Fabrication of such vessels is a specialty that substantially restricts the number of potential suppliers and escalates the cost of supply. The transportation costs are also high because the extreme dimensions of such evaporators cause a requirement for special permits. There is also impact to project schedule since evaporator shipments of such weight are not allowed in some areas due to seasonal road ban restrictions.
A further deficiency is that the fundamental size of the evaporator does not lend itself to modularization. Modularization is a key strategy for minimizing the total installed cost (TIC) of the plant by maximizing the fabrication/assembly shop hours (low unit cost) and minimizing the on-site assembly hours (high unit cost). Since the co-current VTFF evaporators are shipped separately from the balance of the process, they cannot be pre-assembled with pumps, heat exchangers, tanks, piping, and other equipment. This represents additional work that must be performed on-site for a substantial cost.
A final deficiency we note for the technology currently applied is that co-current VTFF evaporators have a center of gravity that is very high above grade (sometimes as high as 60-100 feet). This is a substantial drawback requiring extremely robust civil foundations to be built, which makes their installation even more costly.